There are two types of situations where a side air bag is required to be activated. A situation of a first type can occur when a heavy object such as another vehicle collides with a side part of a vehicle body at a high speed. In this case, the door inner panel intrudes into the passenger compartment ("intrusion") before the vehicle occupant is thrown onto the door by the inertia of the vehicle occupant, thereby causing an injury to the vehicle occupant. A situation of a second type can occur when a heavy object such as another vehicle collides with a side part of a vehicle body at an intermediate speed. In this case, although the intrusion may be relatively insignificant, because of the inertial movement of the vehicle occupant toward the door combined with the intrusion, the vehicle occupant may still hit the inner surface of the door in such a forcible manner that the vehicle occupant may sustain an injury.
To appropriately deal with such different situations, it is conceivable to install a switch such as a compression switch inside the door so as to activate a side air bag in response to the actuation of the switch by an intrusion resulting from a vehicle crash.
However, according to such a proposal, because the detection area that can be covered by each sensor such as a compression switch is relatively limited, it is necessary to install such a large number of sensors in a side part of the vehicle body to deal with a wide range of conceivable situations that a substantial cost will be necessary. Also, when the deformation of the door is relatively small even though the crash speed is high such as when the center pillar (B pillar) of the vehicle body hits a utility pole, expanding the detection area in the door cannot necessarily ensure reliable actuation of these sensors.
It is also conceivable to mount an acceleration sensor to a relatively rigid part of the vehicle body such as a lower part of the center pillar and the side sill, and, with the aim of dealing with a wide range of different side impact modes, monitor a composite acceleration given as a combination of the deformation of the side part of the vehicle body detected by this acceleration sensor and the travel of the overall vehicle body (or the translational movement of the vehicle body) by using a plurality of interval integrators having different integration times (typically carried out as a sliding window integration process by software) as proposed in Japanese patent laid open publication (kokai) No. 4-325349.
According to this proposal, because the acceleration sensor mounted to a side part of the vehicle body is attached to a relatively rigid part of the vehicle body, the single acceleration sensor can deal with impacts over a wide area of a side part of the vehicle body, and can deal with different situations simply by adjusting the integration times of the interval integrators and the determination threshold value through modification of the circuit design and the software, the overall structure can be simplified, and the production cost can be reduced.
For instance, when a heavy object such as another vehicle collides with a side part of a vehicle body at a high speed, and the door inner panel intrudes into the passenger compartment ("intrusion") before the vehicle occupant is thrown onto the door by the inertia of the vehicle occupant, thereby causing an injury to the vehicle occupant (which is referred to as "high speed side impact" hereinafter), such an occurrence can be detected by shortening the integration time (integration interval) to the order of 5 ms, and comparing the result of each interval integration with a prescribed threshold value.
In case of an incidence where a massive object such as another vehicle collides at an intermediate speed with a vehicle side part near the vehicle occupant, and the vehicle occupant is thrown onto the door inner panel or the like due to the combination of the intrusion and the inertial movement of the vehicle occupant toward the door, resulting an injury to the vehicle occupant (intermediate speed side impact), such an incidence can be determined by setting the integration time (integration interval) relatively long (in the order of 10 to 40 ms), and comparing the result of each interval integration with a prescribed threshold value.
However, according to such a side air bag activation control device, because it is unconditionally determined that the side air bag needs to be activated once the result of interval integration of acceleration, which has been monitored, and is given as a combination of the deformation of a vehicle side part and the travel of the entire vehicle, has exceeded a prescribed threshold value, in case of a special mode of impact which would not cause an injury to the vehicle occupant in spite of a significant change in velocity during the interval of integration, an erroneous determination may be made that the side air bag is required to be activated even though it is in fact unnecessary. The "travel" as used herein means the translational movement of the vehicle body.
For instance, when the vehicle has slid sideways and collided with a curbstone or when the vehicle obliquely collides with a curbstone and is bounced back again in an oblique direction, because the impact is produced by the collision of rigid bodies such as the vehicle side wheel or the side sill with the curbstone, even when the speed of the vehicle may be so low that there is no possibility of the vehicle occupant sustaining any injury, because the change in velocity during the integration interval immediately following the impact tends to be significantly high, an erroneous determination may be made that the side air bag is required to be activated even though it is in fact unnecessary if the determination logic for an impact at an intermediate speed is applied. It was also discovered that a low speed collision with a curbstone could produce a sharp change in velocity in a short time interval which is comparable to that of a high speed side impact.
Also, in case of a collision at a side of a front part or a rear part of the vehicle body (which is referred to as an offset side impact hereinafter) which causes little deformation to a side of the passenger compartment could cause a significant change in velocity during the integration interval immediately following the impact even though the speed level is so low that there is no possibility of the vehicle occupant sustaining an injury, an erroneous determination may be made that the side air bag is required to be activated even though it is in fact unnecessary if the determination logic for an impact at an intermediate speed is applied.
Such determination processes are more fully described in the following with reference to FIG. 25. In this drawing, (a) denotes a graph showing a time change of total integration (integration over the entire time interval from immediately after the impact to the elapsing of 100 ms) in a case where the activation of a side air bag is necessary such as a case of an intermediate speed side impact, and (b) denotes a graph showing a time change of total integration (integration over the entire time interval from immediately after the impact to the elapsing of 100 ms) in a case where the activation of a side air bag is not necessary such as a case of a low speed side collision with a curbstone.
In these graphs, t1 indicates the time point by which the determination of impact should be completed, t2 indicates a time point at which the vehicle occupant may sustain an injury, and T indicates the time period required for the deployment of a side air bag. The solid line indicates the values corresponding to the output of the acceleration sensor attached to the collision side of the vehicle body while the broken line indicates the values corresponding to the output of the acceleration sensor attached to the opposite side of the vehicle body.
As shown in FIG. 25(a), in case of a side impact involving a side deformation, the result of total integration of the acceleration on the collision side (impact side) of the vehicle body rises relatively sharply, and continues to rise even after passing the determination time point t1 because of the presence of the component of the deformation of the vehicle body indicated by the shaded area in the drawing. On the other hand, as shown in FIG. 25(b), even in case of a side impact not involving a side deformation such as a collision of a side of a wheel or a side sill with a curbstone, although the speed of impact is relatively small, the result of total integration of the acceleration rises equally sharply, and may reach a level comparable to that of an impact involving deformation.
The interval integration of acceleration which is used for determining an impact may be deemed to be substantially equal to the result of integrating the output G(k) of the acceleration sensor over a prescribed integration time interval .DELTA.T, or to the slope of the curves shown in FIGS. 25(a) and (b). Therefore, as long as the need for activating a side air bag is determined by comparing the result of an interval integration with a single threshold value, it is difficult to distinguish the case of an intermediate speed collision which requires the activation of a side air bag as indicated in FIG. 25(a) and the case of a low speed side collision with a curbstone which does not require the activation of a side air bag as indicated in FIG. 25(b).
In this conjunction, the Inventors have realized that the cases of FIGS. 25(a) and 25(b) can be distinguished from each other if the deformation of a vehicle side part or the deformation of the passenger compartment accommodating the vehicle occupant can be directly determined, and have accordingly arrived at the following lemmas.
With respect to the frequency distribution of the output of an acceleration sensor attached to a vehicle body part which moves in response to a deformation of a vehicle side part as a result of a side impact over a preceding time period, a certain correlation can be found between the two frequency distributions obtained during the time period when the deformation of the vehicle side part is in progress, and during the time period when the deformation of the vehicle side part is not in progress (first lemma).
With respect to the frequency distributions of the outputs of an acceleration sensor attached to a first vehicle body part which moves in response to a deformation of a vehicle side part as a result of a side impact and another acceleration sensor attached to a second vehicle body part which does not substantially move in response to a deformation of a vehicle side part as a result of the side impact over a preceding time period, a certain correlation can be found between the two frequency distributions obtained from the two sensors during the time period when the deformation of the vehicle side part is in progress, and during the time period when the deformation of the vehicle side part is not in progress (second lemma).
It is presumed that these correlations may be attributed to the presence and absence of certain frequency components which are specific to the occurrence of a side impact.